In recent years, there has been a demand to increase the valve diameter of intake and exhaust valves for high-output performance of internal combustion engines. There has also been a demand to provide larger water jackets for efficient cooling of high-output internal combustion engines. In response to these demands, the installation spaces of spark plugs in the internal combustion engines are limited. It is thus required to decrease the diameter of spark plugs.
It is further required that the spark plugs have high ignition performance in order to cope with the strong demand for low emissions from recent internal combustion engines. For the above reasons, the spark plug has a ground electrode of as large dimensions as possible welded to a metal shell even when the metal shell is reduced in diameter. However, the fused joint between the metal shell and the ground electrode decreases in size as the thickness of the ground electrode increases with increasing dimensions and becomes close to the thickness of the metal shell (see for example, Japanese Laid-Open Patent Publication No. 2003-223968). This leads to a deterioration in the joint strength between the metal shell and the ground electrode.
In view of the above problems, it is an advantage of the present invention to provide a spark plug capable of securing the joint strength between a ground electrode and a metal shell even when the spark plug is reduced in diameter.